In voice and data communications networks, there is an on-going need to minimize bandwidth requirements and improve the quality of voice or data traffic. Reducing the bandwidth is typically achieved by implementing compression algorithms to remove redundancy from a signal. On the other hand, signal quality is typically improved by adding redundancy to a signal by, for example, implementing error detection and correction techniques, and by recovering from errors by using lost frame concealment techniques.
Conventional systems attempt to achieve a balance between bandwidth and quality by using a combination of methods. Generally, in a conventional system, at the transmitting side, a source coder/quantizer is provided to quantize and compress the signal to be transmitted, i.e. reduce the bandwidth required, while a channel coder is provided to add information for use in error detection and correction, i.e. improve quality. The signal then travels through a channel (data link) where it may be corrupted. At the receiving side, a corresponding channel decoder, lost frame handler and source decoder are provided to decode the signal received.
One of the issues in communication systems is that, as the interference level increases, the quality of recovered signal falls off rapidly. One conventional approach to overcome this problem has been the use of adaptive source/channel coding (e.g. GSM's Adaptive Multi-Rate (AMR)). Adaptive source/channel coding allows a variation in the level of source coding based on the amount of interference found on the channel data link. For example, a lower level of source coding is performed when the level of interference is high. This allows for more redundancy in the signal and thus, the interference will have less impact on the signal. However, this also has the effect of increasing bandwidth requirements. In a similar way, when the level of interference is low, a higher level of source coding can be used. In this way, adjustments can be made adaptively to counteract the effects of interference during signal transmission.
While adaptive source/channel coding adjusts the source coder based on interference conditions, other conventional approaches are directed to the receiver side of the channel. In a communication system, when a data bit is received, there is some uncertainty as to whether or not the bit is a 1 or a 0 due to distortion, interference, noise on the channel, or the like. In a conventional system, the channel decoder would typically examine an incoming signal and make a decision to determine whether a particular received bit is a 1 or a 0.
A source decoder then receives the bits and processes this data using various well-known techniques depending on the processing performed by the channel decoder to output a recovered signal. However, prior to processing by the source decoder, lost frame concealment techniques are employed to deal with frames of data that are lost or otherwise damaged.